Charged particles surround the Earth. Perhaps the most widely known of these particle populations is the eponymous Van Allen radiation belts, discovered in the 1950's by the physicist from Iowa. However, there exists a plethora of charged particle populations surrounding the Earth. Physicists call them by names like "ring current" and "plasmasphere". They are trapped by the Earth's magnetic field and are part of what scientists call the Earth's "magnetosphere."
      The magnetosphere, except under very unusual conditions, envelops the region of space known as geosynchronous orbit. It is here where many of our expensive telecommunications and other satellites reside. These satellites are particularly vulnerable to space weather events, geomagnetic storms, which can incapacitate or destroy them. By understanding the physics of the magnetosphere, space physicists are attempting to predict the occurrence and behavior of these storms so that in the future action can be taken to reduce the malevolent effect of space weather events on our space-borne assets.
      LENA's main role in this effort is to study a particular particle population known as ionospheric outflow. This population is a sporadic stream of protons and oxygen ions flowing from the Earth's ionosphere, which is just above the Earth's surface. These charged particles are confined and guided by magnetic field lines. In order to observe them with traditional instrumentation, the spacecraft and its instruments must actually be inside of the population.
      Every once in a while, though, these charged protons and oxygen ions interact with a neutral atom in the Earth's atmosphere and steal its charge, becoming neutral. Once neutral, the outflowing ions are no longer confined to a magnetic field line and can flow freely for long distances from their original location. LENA observes these neutral products of the ion outflow to image this process from a distance in much the same way an astronomer uses light to descry far-away planets.
      The figure below shows an example of a LENA image of ionospheric outflow from the Earth's poles. The figure displays neutral hydrogen and oxygen resulting from charge exchange with the Earth's atmosphere. Each of the three images covers a bit over 70% of the sky. On the Earth, the auroral oval, home of the northern and southern lights, is indicated in red. The other lines and labels correspond to the Earth's magnetic field.

      However, applications of neutral atom imaging are not confined to regions close to the Earth. LENA has observed other populations of neutral atoms that originate outside of the Earth's magnetosphere. One of these is interstellar neutral atoms which come from beyond the farthest edges of our solar system. Another is neutral solar wind which forms when the hot ionized gas flowing from our Sun charges exchanges with neutral atoms between the Sun and the Earth. Low energy neutral atom imaging could resolve mysteries at other planets, as well, for example, why Venus lost its water and whether or not conditions at Europa, Jupiter's moon, are consistent with life. The Mars Express/ASPERA-3 instrument is already making measurements at the Red Planet. With this tool, low energy neutral atom imaging, in hand, scientists can now see what was previously invisible.